The present invention relates to a method of conducting a process involving the generation of steam in which the steam is raised from a hot process stream that is passed into a steam generator. More particularly, the present invention relates to such a method in which hot process stream is introduced into a thermoacoustic engine to recover work prior to its being introduced into a steam generator.
There are a variety of industrial processes in which hot process streams are generated that are used to raise steam. As will be discussed the steam can be used to generate additional energy that is useful for the process or that can be converted to electricity to be reintroduced into a power grid. Moreover, the steam itself may constitute a necessary ingredient of the process.
For instance, heat recovery steam generators, also known as waste-heat boilers, recover energy from process streams produced by such processes as incineration systems, cogeneration systems and chemical process plants. Incineration systems operate at high temperatures, typically between 1650 to 2,4000xc2x0 F. In such systems, waste heat recovery from process streams formed of heated flue gases can be applied to generate stream that in turn can be used to generate electricity. In cogeneration plants thermal energy contained in a process stream formed from a gas turbine exhaust can also be recovered in the form of steam and electricity. In chemical process plants, steam generators are often used to cool process streams. The steam generated can be used to recover energy or can itself be used in the process being conducted.
A concrete example of a chemical process involving steam generation can be found in steam-reforming operations in which product streams from the reformer and shift converter are cooled. A mixture of feed gas and steam is fed into a reaction furnace or reformer heated by flue gases. The product stream is cooled before being sent to a reactor where an exothermic shift conversion reaction takes place. The product stream from the shift conversion also needs to be cooled. As may be appreciated, during the cooling stages, steam may be raised to recover energy and to serve as a reactant.
The foregoing involve just a few exemplars of many processes that generate high temperature process streams in which energy can be recovered by steam generators. In any process in which energy is to be recovered at high temperature through steam generation, thermodynamic inefficiencies can arise from the rejection of heat from the cold end of steam generators of practical size and cost.
As will be discussed, the present invention involves the integration of a process in which steam is generated with a thermoacoustic engine. The processes can be of the types described above. In this regard, the operation of thermoacoustic engines is fully described in Physics Today, xe2x80x9cThermoacoustic Engines and Refrigeratorsxe2x80x9d, by Gregory W. Swift, pp. 22-27, July 1995. Briefly stated, a thermoacoustic engine is a known device that employs a resonator tube containing hot and cold end heat exchangers thermally linked by a stack of parallel plates to convert thermal energy to acoustic energy. The work of the acoustic energy can be converted to work, electricity, or refrigeration. Examples of devices in which thermoacoustic work is converted to electricity are shown in U.S. Pat. Nos. 5,996,345 and 4,559,551. An example of a device in which thermoacoustic work is used to generate refrigeration is described in U.S. Pat. No. 4,953,366 in which a thermoacoustic engine is used in combination with an orifice pulse tube refrigerator. It has at least been proposed to use the combination of a thermoacoustic engine and orifice pulse tube refrigerator, known as a TADOPTR, in natural gas fields to liquefy natural gas. In such an application, a part of the natural gas to be liquefied is burned to power a TADOPTR that in turn is used to liquefy a remaining part of the natural gas.
In the present invention, a portion of the energy is advantageously recovered from a high temperature process stream within a thermoacoustic engine prior to the generation of steam through indirect heat exchange with an intermediate temperature process stream at a lower temperature. As a result, the overall thermodynamic efficiency of the process and therefore the amount of energy able to be recovered is increased.
The present invention relates to a method of conducting a process involving the generation of steam. In accordance with the invention, a hot process stream is generated. Heat is transferred from the hot process stream to a thermoacoustic engine to recover energy from the hot process stream as thermoacoustic work and to generate an intermediate temperature process stream. The intermediate temperature process stream is introduced into a steam generator to generate the steam.
As may be appreciated, from considerations of Carnot cycle efficiency alone, in order to recover the greatest possible energy from a high temperature process stream through steam generation, a quite massive, if not expensive, heat exchanger is required to reject heat from the process at the lowest temperature possible, normally ambient temperature. By recovering energy first in a thermoacoustic engine and then through the steam generator, a more ideal efficiency is approached because energy recovered in two stages, namely, at high temperature, in a thermoacoustic engine, and then at a lower temperature, within the steam generator, through heat exchange with the cooler intermediate process stream. After heat exchange within the steam generator, the intermediate process stream can be discharged at a temperature that is practically lower than that obtainable had the high temperature process stream been directly introduced into the steam generator. In this regard, a synergy is realized because the thermoacoustic engine requires a high temperature for its operation while the steam generator can adequately function at a lower temperature.
Other advantages can be realized by the method of the invention. A hot process stream can be generated by burning a fuel in the presence of an oxidant. In such case, carbon dioxide is generated. The steam generator can cool the intermediate temperature process stream into a cool stream and carbon dioxide produced from the burning of the fuel can be recovered from the cooled stream. The advantage of this is that the carbon dioxide can be sequestered to prevent the formation of greenhouse gases and can later be sold or used for further industrial processes. Preferably, a water stream can be heated in direct heat exchange with the working fluid within the thermoacoustic engine, the cooled stream during the recovery of carbon dioxide and in the steam generator to produce the steam. The work may be recovered by introducing the steam into a steam turbine. A stream of carbon dioxide can be liquefied in an acoustic refrigerator driven by the thermoacoustic engine.
The foregoing advantageous applications can be used in such industrial processes as steam methane reforming. For instance, the steam can be combined with a methane containing feed and subjected to steam methane reforming, thereby to produce a hydrogen containing gas. The hydrogen can be separated from the hydrogen containing gas to produce a hydrogen product stream and the fuel. The hydrogen product stream can be liquefied in an acoustic refrigerator driven by the thermoacoustic engine.
Another potential application is in coal gasification. In such application, further energy may be recovered from the hot process stream within a turbine prior to the heat transfer with the thermoacoustic engine. The steam can then be introduced into a coal gasifier to generate part of the fuel. The further energy can be applied to power an air compressor to produce a compressed air stream that can be used to at least in part form the oxidant. Preferably the coal gasifier produces an untreated fuel stream and part of the compressed air stream forms the oxidant. The untreated fuel stream can be introduced in sequence into a further thermoacoustic engine to produce thermoacoustic work, an exhaust gas cooling unit, a clean up unit to produce the part of the fuel. The further thermoacoustic work can be extracted from the thermoacoustic engine.
The heat can be transferred from the hot process stream to the thermoacoustic engine by a heat transfer fluid heated through indirect heat exchange with the hot process stream produced as a flue gas within a furnace. The furnace can be a blast furnace and the steam can be used to drive a steam turbine thereby to produce shaft work. The shaft work can be applied to two air compressors to compress air. One of the two air compressors is coupled to a vacuum pressure swing adsorption unit to produce an oxygen stream and the other of the two air compressors produces a compressed air stream. The compressed air stream and the oxygen stream are heated within the blast furnace. The compressed air stream and the oxygen stream after having been heated can be introduced into the blast furnace for combustion of coke and thereby to produce a reducing gas.
In a still further application, the oxygen containing gas can be passed into an oxygen transport membrane reactor, thereby to produce a heated retentate stream and an oxygen product stream. The hot process stream is then formed at least in part from the heated retentate stream. The oxygen product stream can be liquefied in an acoustic refrigerator driven by the thermoacoustic engine.
A yet still another application involves cryogenic air separation. A hot process stream can be produced from the exhaust of the gas turbine. Shaft work produced by the gas turbine can be applied to a compressor to compress air in a cryogenic air separation unit. The cryogenic air separation unit produces at least one product stream, predominately composed of nitrogen or oxygen and the at least one product stream can be liquefied within an acoustic refrigerator driven by the thermoacoustic engine. In such an application, the steam can be introduced into a steam turbine to produce further shaft work.
In a further application of the present invention, the fuel and oxidant are burned within a natural gas engine generating shaft work. Part of the shaft work is used to drive an air compressor to compress air in a cryogenic air separation unit. The cryogenic air separation unit produces a product stream enriched in one of oxygen and nitrogen. Such product stream is introduced into a vapor compression refrigerator driven by a further part of the shaft work and then into an acoustic refrigerator driven by the acoustic work produced in the thermoacoustic engine.